The present invention relates to a control apparatus for a direct-injection engine that reduces HC exhausted immediately after a direct-injection engine is started.
HC emission while a catalyst is still inactive immediately after cold start of an engine accounts for a very large portion of total emissions. In order to reduce HC emissions at this timing, the following method is used conventionally; the ignition timing is retarded after the top dead center of compression to raise the exhaust gas temperature so as to activate a three-way catalyst earlier.
However, this method suffers the following two problems.
(1) Since the ignition timing is retarded considerably, fuel economy suffers, and also combustion stability suffers due to a cold period.
(2) Although the period until the catalyst begins to be partially activated and an HC purifying ratio begins to rise rapidly (to be referred to as partial light-off hereinafter) is shortened, HC (RawHC) emitted by the engine during this period is exhausted unpurified, and a scheme for reducing RawHC emitted by the engine during the period until partial light-off is required.
Also, the following technique has been proposed. That is, fuel is injected in two injections, i.e., an intake stroke and compression stroke while the catalyst remains inactive, and stratified air-fuel mixtures are formed, i.e., an air-fuel mixture richer than the stoichiometric air-fuel ratio is formed around a spark plug and a leaner air-fuel mixture is formed around the former air-fuel mixture, thereby promoting warming up of the catalyst, and assuring high combustion stability. More specifically, this reference describes that the air-fuel ratio is set at xcex≈1, the injection amount of the intake stroke is set to be equal to or larger than that of the compression stroke, the ignition timing is retarded, and swirl is generated (Japanese Patent Laid-Open No. 10-212987). U.S. Pat. No. 6,044,642 discloses that fuel injection is divided to a trailing injection on a compression stroke and a leading injection earlier than the trailing injection, and a swirl is strengthened so as to activate the catalyst earlier.
However, when a swirl is strengthened before the catalyst warms up, since it has an effect of raising the burning rate, high combustion stability can be assured upon retarding the ignition timing, but an afterburning effect suffers, resulting in an insufficient RawHC reduction effect during the period until partial light-off.
The present invention has been made in consideration of the aforementioned problems, and has as its object to provide a control apparatus for a direct-injection engine, which can shorten the period required until light-off at a predetermined HC purifying ratio at which at least the HC purifying ratio begins to rise before a catalyst warms up, reduces RawHC emitted by the engine before light-off to the predetermined HC purifying ratio, and can minimize deterioration of fuel economy.
In order to solve the above problems and to achieve the above object, according to the first aspect of a control apparatus for a direct-injection engine of the present invention, the air-fuel ratio in a combustion chamber is set to be xcex≈1 before the catalyst warms up, and a varying means operates to divisionally inject fuel during at least a former period from the start of an engine until a catalyst, which is halfway through the catalyst temperature rise before the catalyst warms up, is partially activated, and to set an intake flow strength in the former period to become lower than that in the latter period halfway through the catalyst temperature rise after the former period. In this way, combustion becomes slow to promote afterburning so as to shorten the period required until the predetermined light-off state in which the HC purifying ratio of the catalyst begins to rapidly rise, thus suppressing deterioration of fuel economy and reducing RawHC emitted by the engine before the predetermined light-off state.
According to the second aspect, the varying means increases the intake flow strength while divisionally injecting fuel even during the latter period. In this manner, deterioration of fuel economy can be suppressed while promoting warming up of the catalyst.
According to the third aspect, the former period is a period required until the activation state of the catalyst reaches an HC purifying ratio approximately half a maximum HC purifying ratio of the catalyst itself. As a result, even when the intake flow strength is increased later, sufficiently high HC purification is assured, and warming up of the catalyst can be promoted while suppressing deterioration of fuel economy and assuring high HC purification.
According to the fourth aspect, the varying means operates before the catalyst warms up in a low engine-speed range. In this manner, the amount of RawHC emitted by the engine before the catalyst warms up can be reduced.
According to the fifth aspect, the former period with a low intake flow strength takes a trailing injection timing retarded with respect to that in the latter period with a high intake flow strength. In this fashion, combustion stability can be assured by suppressing fuel mist from scattering, and the exhaust gas temperature can be rapidly raised by slow combustion.
According to the sixth aspect, when fuel is injected in two injections in the former period, the fuel injection amount in leading injection is set to be smaller than that in trailing injection. In this manner, the exhaust gas temperature rise effect and HC and NOx reduction effects are obtained while assuring combustion stability.
According to the seventh aspect, the fuel injection amount in leading injection is set to be xc2xc or more of the total injection amount. As a result, the exhaust gas temperature rise effect and HC and NOx reduction effects are obtained while assuring combustion stability.
According to the eighth aspect, fuel is two-divisionally injected in intake and compression strokes. In this way, the exhaust gas temperature rise effect and HC and NOx reduction effects are obtained while assuring combustion stability.
According to the ninth aspect, the air-fuel ratio in a cylinder in the former period is set to be leaner than that in the latter period within the range of xcex≈1. As a result, ignitability drop due to offset of a rich air-fuel mixture around the spark plug during the former period with a low intake flow strength can be prevented. In addition, since the air-fuel ratio in the entire cylinder is slightly leaner than xcex=1, the RawHC emission amount emitted by the engine during the former period with a low HC purifying ratio can be reduced.
According to the 10th aspect, the air-fuel ratio in a cylinder during the former period is set to be close to but leaner than xcex=1 before beginning of o2 feedback, and is set at xcex=1 after beginning of o2 feedback. In this manner, the purification efficiency of a three-way function of the catalyst (three-way catalyst, NOx catalyst, or the like) can be improved by setting the air-fuel ratio at xcex=1 after beginning of o2 feedback, while reducing RawHC emission from the engine.
According to the 11th aspect, a feedback reference value upon o2 feedback during the former period is set to be leaner than that upon o2 feedback during the latter period. As a result, RawHC emission amount reduction and suppression of fuel economy deterioration can be achieved while maintaining the three-way function of the catalyst.
According to the 12th aspect, the ignition timing is retarded with respect to an identical load and identical engine speed after the catalyst warms up. In this manner, while afterburning due to slow combustion can be advanced earlier than normal setting to promote warming up of the catalyst, reducing RawHC emission from the engine.
According to the 13th aspect, a swirl is generated in a cylinder so that the local air-fuel ratio around a spark plug becomes rich by trailing injection, and the varying means changes the swirl ratio in the cylinder. As a result, since a rich air-fuel mixture can be surely locally present around the spark plug, a variation factor of an indicated mean effective pressure Pi (Pi variation factor=Pi standard deviation "sgr"/Pi cycle mean valuexc3x97100 (%)) in a stratified engine can be reduced, thus suppressing deterioration of combustion stability.
According to the 14th aspect, the spark plug is disposed on the central upper end portion of the cylinder, a fuel injection valve is disposed on the peripheral upper end portion of the cylinder, and a stratification cavity is formed on the top of a piston near the fuel injection valve. With this arrangement, since a rich air-fuel mixture can be surely locally present around the spark plug, the Pi variation factor can be reduced in the stratified engine, thus suppressing deterioration of combustion stability.
According to the 15th aspect, the air-fuel ratio in a combustion chamber is set to be xcex≈1 before the catalyst warms up, and an intake flow control valve operates to divisionally inject fuel during at least a former period from the start of an engine until a catalyst, which is halfway through the catalyst temperature rise before the catalyst warms up, is partially activated, and to set an intake flow strength in the former period to become lower than that in the latter period halfway through the catalyst temperature rise after the former period. In this way, combustion becomes slow to promote afterburning so as to shorten the period required until the predetermined light-off state in which the HC purifying ratio of the catalyst begins to rapidly rise, thus suppressing deterioration of fuel economy and reducing RawHC emitted by the engine before the predetermined light-off state.
Other objects and advantages besides those discussed above shall be apparent to those skilled in the art from the description of a preferred embodiment of the invention which follows. In the description, reference is made to accompanying drawings, which form a part thereof, and which illustrate an example of the invention. Such example, however, is not exhaustive of the various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention.